I’ve got a couple of cheap cordless drills, one’s a 9.6 volt and the other is 18.2 volts. They work well, but the annoying thing is the charger isn’t “smart”. By smart, I mean that it will continue to charge the battery even after it’s been charged. With NiCad batteries this means that the batteries become damaged after time and stop holding a charge.

I had started to build my own smart charger but made one major mistake with the MOSFETs so I threw out that design and went simpler. I converted the chargers that came with the batteries to trickle chargers based on some advice from a coworker who pointed out that more often than not, I just want a charged drill when I need it, and rarely will need to charge the battery quickly.

Rechargable batteries are rated in terms of Amp-Hours (AH) or milliamp Hours (mAh). Theoretically, a battery rated at 1AH can deliver 1A of current for 1 hour, or 2A for 1/2 hour, or 1/2A for 2 hours, and so forth.

The product of Amps and time is the Coulomb, so the capacity of the battery is sometimes written as C. To charge the battery one must put back the number of coulombs that were taken out (plus some for transfer loss). Charge rates are usually expressed as a rate of C. Most cheap drills I’ve run into have battery packs rated at 1200mAh. To charge the drill I could do 1.2A for 1 hr (which would be a rate of C), or .6A for 2 hrs (C/2), or 2.4A for 1/2 hr (2C). Practically speaking though, the cheap batteries need a lower charge rate for a longer time, such as C/2 or C/4.

The chargers that come with the drills are actually just a transformer to limit the current. The only electronics are a transistor, a resistor, and an LED to indicate the current is flowing. The transformer limits the current, so if you have a 300mA transformer you need 4-5 hours of charging at C/4. Leave it any longer at that rate, and you’ll be hurting your batteries.

Drop the rate really low, to C/20 (~60mA) and you can keep the current on indefinately. It will take a long time to charge (20-24 hours), but you never have to worry about pulling the drill off. This is how most smart chargers work, they give the battery a high charge until it’s done (ie at C, or C/2), and then drop to a low charge like C/20 to keep the battery topped off.

Converting an existing charger to a trickle charger is actually very simple. There is a part called the LM117 that’s an ajustable 3 terminal regulator. Normally the LM117 is used to provide a constant voltage given a variable input, but it can do other things. The circuit I used is even in the datasheet, it makes the device a 50mA constant current source. That is, no matter what voltage you put in, the LM117 will pump out that voltage at 50mA. You have to apply 24 ohms across terminals 1 and 2 (read the datasheet for which pin is which), for which I used two 47ohm resistors in parallel soldered directly to the lm117. Throw that in between the battery and the source, and you’ve got a constant current source.

The parts are easily available and should cost no more than a few dollars. You might even be able to order the LM117s as samples, or go to a local hobby shop to get it. Glitchbuster is a great place to order stuff from too.

Be careful when working with batteries. This only works for NiCad and possibly NiMH, and not LiON, and certainly not alkeline.